The present invention relates to a non-aqueous electrolyte having excellent cycle characteristics, electric capacity, and high-temperature storage properties during charging, as well as a non-aqueous secondary battery using the electrolyte.
A non-aqueous electrolyte-containing secondary battery using an alkaline metal, such as lithium (Li) or sodium (Na), for its negative electrode has a high electromotive force and is expected to have a higher energy density than those of conventional nickel-cadmium storage battery and lead acid battery. Especially there have been extensive studies on the non-aqueous electrolyte-containing secondary battery using Li for the negative electrode.
Application of an alkaline metal for the negative electrode causes dendrite in the process of charging, and the resulting battery is liable to be short-circuited and has low reliability. One proposed technique uses an alloy of Li and aluminum (Al) or lead (Pb) for the negative electrode.
Application of this alloy for the negative electrode causes Li to be stored in the alloy of the negative electrode in the course of charging. This effectively prevents the occurrence of dendrite and gives the battery having higher reliability.
The discharge potential of this alloy is, however, noble by approximately 0.5 V relative to the discharge potential of metal lithium. This leads to a decrease in voltage of the battery by 0.5 V and lowers the energy density of the battery.
The technique of using an intercalation compound of Li and carbon (C) such as graphite for the negative electrode active material has been studied, and the battery based on this technique has been put to practical use under the trade name of lithium ion secondary battery.
No dendrite occurs in the negative electrode of the intercalation compound, since Li enters between the layers of C by charging. The discharge potential of this negative electrode is only slightly noble by approximately 0.1 V relative to the discharge potential of metal lithium. This negative electrode is preferable, because of the less decrease in voltage of the battery.
In the case of graphite, the quantity of Li entering between the layers of C by charging is theoretically only an amount to allow formation of C6Li at the maximum. The electric capacity under the condition of the maximum quantity of Li is 372 Ah/kg. Carbon materials, diverse alloys, metal oxides and the like having low crystallinity have accordingly been proposed to give the higher capacity than the theoretical value.
With an improvement in capacity and other related performances of the non-aqueous electrolyte-containing secondary battery, the important technical issue has shifted to enhance the stability and the durability in a high-temperature environment of the battery.
Some proposed techniques add a phosphoric ester to the electrolyte from the viewpoint of improving the flame retardant properties and autolysis of the non-aqueous electrolyte-containing secondary battery or especially of the electrolyte and thereby enhancing the stability of the non-aqueous electrolyte-containing secondary battery (for example, JAPANESE PATENT LAID-OPEN PUBLICATION No. 4-184870, No. 8-111238, No. 9-180721, and No. 10-55819). One concrete example is a non-aqueous electrolyte-containing secondary battery, in which a large quantity of a tri-substituted phosphoric ester having a substituent of 4 or less carbon atoms is included in the electrolyte.
Application of an electrolyte containing a large quantity of a general phosphoric ester such as triethyl phosphoric ester improves the stability of the battery, because of the flame retardant properties of the electrolyte.
The phosphoric ester, however, tends to be easily reduced and decomposed on the negative electrode. The type and content of the phosphoric ester may adversely affect the performances of the battery using the phosphoric ester, for example, the energy density, the cycle characteristics, the high-efficiency discharge characteristics, and the high-temperature storage properties. In order to solve this problem, one proposed technique restricts the quantity of a phosphoric ester added to the electrolyte (JAPANESE PATENT LAID-OPEN PUBLICATION No. 8-22839).
The material for the negative electrode having a greater capacity than the carbon material such as graphite has given the non-aqueous electrolyte-containing secondary battery having a large discharge characteristic. The electrode accordingly accumulates the greater quantity of electricity per unit weight or unit volume. With a progress of the charge-discharge cycle, the non-aqueous electrolyte is decomposed to form a passive film on either the positive electrode or the negative electrode, which gradually lowers the discharge capacity.
While the battery is stored in the charged state, the decomposition reaction of the non-aqueous electrolyte arises on either the negative electrode or the positive electrode at high temperatures, because of the high voltage of the battery. There is also deterioration due to the thermal reaction of the non-aqueous electrolyte, which significantly lowers the properties of the battery after storage at high temperatures.
The proposed techniques JAPANESE PATENT LAID-OPEN PUBLICATION No. 8-111238, No. 9-180721 and No. 10-189038 have not actively mentioned the improvement in electrochemical stability of the non-aqueous electrolyte by addition of the phosphoric ester. There has also been no specific discussion on the reactivity of the non-aqueous electrolyte to the positive electrode while the battery is kept at high temperatures. The prior art techniques accordingly have not suggested any countermeasures to solve the above-mentioned drawbacks.
In order to solve the above problems, the inventors of the present invention have intensively studied the phosphoric ester to be included in the non-aqueous electrolyte and have found that application of a non-aqueous electrolyte including a halogen atom-containing phosphoric ester derivative to a non-aqueous secondary battery significantly improves the cycle characteristics and the high-temperature storage characteristics.
The object of the present invention is thus to enhance the electrochemical stability of a non-aqueous electrolyte and suppress a side reaction on either of a positive electrode and a negative electrode by making a phosphoric ester derivative included in the non-aqueous electrolyte.
More specifically, the object of the present invention is to provide an electrolyte for a non-aqueous battery and a non-aqueous secondary battery, which effectively prevents deterioration due to the thermal reaction of the non-aqueous electrolyte, thereby reducing a decrease in discharge capacity with a progress of the charge-discharge cycle and deterioration of the high-temperature storage characteristics.
The present invention is thus directed to an electrolyte for a non-aqueous battery, comprising:
a non-aqueous solvent;
a solute; and
at least one selected from the group consisting of phosphoric ester derivatives expressed by Formula (1) 
where R1a and R2a independently denote aliphatic hydrocarbon groups having 1 to 12 carbon atoms and X denotes a halogen atom, and by Formula (2) 
where R1b denotes an aliphatic hydrocarbon group having 1 to 12 carbon atoms and X denotes a halogen atom.
In Formulae (1) and (2), it is preferable that X represents a fluorine atom.
It is also preferable that the electrolyte contains 0.001 to 10% by weight of the phosphoric ester derivative.
It is further preferable that the non-aqueous solvent comprises a cyclic carbonate and a chain carbonate, and the solute is a Li salt.
The present invention is also directed to a non-aqueous secondary battery comprising a rechargeable (chargeable and dischargeable) positive electrode, a rechargeable negative electrode, and the electrolyte discussed above.
It is preferable that the rechargeable negative electrode contains at least one active material selected from the group consisting of graphite, metal lithium, Li alloys and Li compounds, the alloy and compounds containing at least one of Sn, Si, Al, and In.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.